1. Field of the Invention
The present invention relates to a method for manufacturing a fitting including a blind nut that is, e.g., a fitting for hanging a TV main body on a wall and to a tap tool for use in this method.
2. Description of the Related Art
As a fitting for hanging a TV main body on a wall, there is a fitting including a blind nut. This fitting is formed by integrally coupling a portion that is a fitting plate 1 with the other portion that is a blind nut 2 as shown in FIG. 7. A screw fastening hole 1b is formed in the fitting plate 1, and a plastic cover provided on a rear side of a flat-screen TV is screwed from an inner side. This plastic cover has a hole, the blind nut 2 is placed in this hole, and the blind nut 2 is screwed when hanging the TV on the wall by using a screw. The blind nut 2 is adopted because screwing can be effected on a bottom of the blind nut even if a final user uses a screw that is long more than necessity.
The fitting plate 1 has a board thickness of 0.5 to 4.0 mm, has an electrogalvanized steel sheet (SECC) or the like as a material, is subjected to processing (bending and boring) by a pressing machine having a die in advance, and has an insertion hole la for the blind nut 2 formed in a flat plate portion thereof.
As shown in FIG. 6, the blind nut 2 of M6 is inserted into the insertion hole la formed in the fitting plate 1, and caulked and fixed (fastened and firmly attached) by using a pressing machine to be provided as a product.
A cost of the blind nut 2 is high, and it accounts for a substantial fraction of a total cost.
Thus, a method for integrally forming the blind nut 2 from the fitting plate 1 has been examined. According to this method, a portion corresponding to the fitting plate 1 is formed into an embossed shape by drawing processing, and an internal diameter portion is subjected to tap processing to integrally form the portion corresponding to the blind nut 2.
Meanwhile, for the purpose of reducing the number of components, a technology that performs burring processing (drawing processing effected by the pressing machine) with respect to a bracket and carries out thread cutting processing (tapping) with respect to an inner periphery of the thus formed cylindrical portion to integrally form a female screw that substitutes for a nut with the bracket has been proposed, and there are the following patent documents which have been filed and disclosed in Japan.
[Application Number] Japanese Patent Application No. 2002-321528
[Publication Number] Japanese Patent Application Laid-open No. 2004-154802
This is, as shown in FIG. 8, a method for forming an integral female screw portion in a press molded article by which a panel material 110 is subjected to press working to form the female screw portion integrated with this panel material 110, and a cylindrical portion 111 formed by performing drawing processing with respect to the panel material 110 is repeatedly subjected to compression processing in each of a radial direction and a height direction thereof to increase a wall thickness of the cylindrical portion 111, and then thread cutting is effected with respect to an inner periphery of this cylindrical portion 111.
It is to be noted that the wall thickness of the cylindrical portion constituting the female screw portion is equal to a board thickness of a bracket or it is smaller than the board thickness of the bracket depending on a drawing depth at the time of burring processing, whereby strength of the female screw portion is lowered to degrade fastening power. As a measure for an increase in cost and weight which occurs when the board thickness of the bracket is increased to assure the strength of the female screw portion, repeating the compression processing along each of the radial direction and the height direction of the cylindrical portion formed by effecting drawing processing to the panel material to increase the wall thickness of the cylindrical portion and then carrying out the thread cutting with respect to the inner periphery of this cylindrical portion has been proposed.
FIGS. 9 to 14 show manufacturing processes. As shown in FIGS. 9 and 10, a draw portion in a workpiece shape is determined as a cylindrical portion 111 having a bottom. An angle of each corner in a draw end portion 111a and a root portion 111b of the cylindrical portion 111 is substantially set to a right angle. As a result, buckling or material slipping of the cylindrical portion 111 caused due to the compression processing can be avoided. Forming this cylindrical portion 111 by the single drawing processing is ideal, but it is impossible in practice, and hence the processing is carried out at a plurality of steps.
As shown in FIG. 11, a bottom plate of the draw end portion 111a in the cylindrical portion 111 is punched out, the cylindrical portion 111 having this workpiece shape is compressed along each of a radial direction and a height direction (a draw depth direction) thereof, and each corner portion in the root portion 111b of the cylindrical portion 111 is determined by this processing.
As shown in FIG. 12, the compression forming is carried out, the cylindrical portion 111 is further compressed in the radial direction and the height direction, and the wall thickness of the cylindrical portion 111 is increased.
As shown in FIG. 13, the compression processing is further carried out, the cylindrical portion 111 is thereby further compressed, and it is processed into a shape optimum for thread cutting processing (tapping) as a subsequent process, and a product having a screw portion 111c formed on an inner hole of the cylindrical portion 111 can be obtained as shown in FIG. 14.
As described above, when the wall thickness of the cylindrical portion 111 is increased and then the screw portion 111c is processed to form the female screw portion, strength of the female screw portion can be enhanced without increasing the board thickness of the panel material 110. Further, in the compression processing for increasing the wall thickness of the cylindrical portion 111, when the root portion 111 of the cylindrical portion 111 is formed into a tapered shape as depicted in FIG. 13, the strength of the female screw portion that is finally formed can be further improved.
Moreover, U.S. Pat. No. 5,503,596 “METHOD OF MANUFACTURING TEE NUTS” by Yutaka Nagayama and Japanese Applications [Japanese Patent Application No. 112641-1993] (Publication No. 323315-1994) and [Japanese Patent Application No. 268820-1993] (Japanese Patent Application Laid-open No. 116769-1995) likewise disclose a T-nut 1 that includes a shaft portion and a flange portion which are formed of an integral metal material, the flange portion outwardly protruding from a first end portion of this shaft portion, the shaft portion having a hollow cylindrical shape, a female screw being formed on an entire inner peripheral surface of the shaft portion.
It is to be noted that, in this patent document, a wall thickness of a cylindrical portion constituting the female screw portion is equal to a board thickness of a bracket, or it is smaller than the board thickness of the bracket depending on a draw depth at the time of burring processing, whereby the female screw portion has low strength and poor fastening power. As a measure for an increase in cost and weight which occurs when the board thickness of the bracket is increased to assure the strength of the female screw portion, repeating the compression processing along each of a radial direction and a height direction of the cylindrical portion formed by effecting drawing processing to a panel material to increase the wall thickness of the cylindrical portion and then carrying out a thread cutting processing with respect to the inner periphery of this cylindrical portion has been proposed.
Meanwhile, when these schemes are applied to manufacture of the above-described fitting for hanging a TV main body, the blind nut 2 is formed into an embossed shape by drawing processing, and an internal diameter portion thereof is tapped to be formed.
Therefore, whether the tap processing has been carried out can be judged by visually confirming a portion near a bag-like inlet portion of a product, but an inspection for a depth of tap processing cannot be visually conducted. Therefore, a screw gauge or a screw must be screwed into a product, and a difference in length of the screw gauge or the screw at this moment must be measured to determine a processing depth.
This method takes time since the gauges are put into products one by one for the determination, and it is unsuitable for determining many products.
In view of the above-described drawbacks, it is an object of the present invention to provide a method for manufacturing a fitting including a blind nut that enables visually confirming whether the tap processing has been effected from the outer side and also provide a tap tool for use in this method.